Before you begin, initialize the solution to enable creation of this streamline.
To create a constrained streamline, right-click the Derived Parts node and select New Part > Constrained Streamline...
Typically, an in-place dialog would be used for this process, and it would appear as soon as you execute this pop-up menu action.
However, if the conditions for activating this dialog are not met, then this action causes the new constrained streamline part to be added to the simulation tree immediately, and you can define the part using the properties.